The present invention relates to the capacitor arts. It finds particular application in conjunction with titanium, aluminum, tantalum and other metal sponges for capacitor anodes grown in the form of dendrites on metallic substrates, and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to the growth of sponges for a variety of applications in which a high accessible surface area to volume ratio is desired.
Electrical devices, such as power supplies, switching regulators, motor control-regulators, computer electronics, audio amplifiers, surge protectors, and resistance spot welders often require substantial bursts of energy in their operation. Capacitors are energy storage devices that are commonly used to supply these energy bursts by storing energy in a circuit and delivering the energy upon timed demand. Typically, capacitors consist of two electrically conducting plates, referred to as the anode and the cathode, which are separated by a dielectric film. In order to obtain a high capacitance, a large dielectric surface area is used, across which the electrical charge is stored. The capacitance, C of a capacitor is determined by the formula:                               C          ⁡                      [            Farads            ]                          =                              Q            ⁡                          [              coulombs              ]                                            V            ⁡                          [              volts              ]                                                          (        1        )            
where Q is the electrical charge and V is the voltage between the plates. Capacitance is proportional to the charge-carrying area of the facing plates, A, and is inversely proportional to the gap width, X, so that                               C          ⁡                      [            Farads            ]                          =                              (                          ϵ              ·                                                ϵ                  0                                ⁡                                  [                                      F                    /                    m                                    ]                                                      )                    ⁢                                    A              ⁡                              [                                  m                  2                                ]                                                    X              ⁡                              [                m                ]                                                                        (        2        )            
where (xcex5xc2x7xcex50) is a proportionality constant, xcex50 is the permittivity of vacuum (value=8.85xc3x9710xe2x88x9212 Farad/m), and xcex5 is the relative permittivity or dielectric constant for a dielectric substance. High capacitance capacitors should have a large area, A, and a thin dielectric film with a high dielectric constant.
Commercial capacitors attain large surface areas by one of two methods. The first method uses a large area of thin foil as the anode and cathode. See, e.g., U.S. Pat. No. 3,410,766. The foil is either rolled or stacked in layers. In the second method, a fine powder is sintered to form a single slug with many open pores, giving the structure a large surface area. See, e.g., U.S. Pat. No. 4,041,359. Both these methods require considerable processing to obtain the desired large surface area. In addition, the sintering method results in many of the pores being fully enclosed and thus inaccessible to the dielectric.
Metallic sponges provide an opportunity for increasing the surface area over conventional capacitor materials. Metallic sponges of titanium, such as those produced by the Hunter and Kroll processes, have relatively large surface areas. However, due to the random growth patterns, surface areas are not maximized and a considerable portion of the surface is inaccessible, being fully enclosed by the sponge. Additionally, chemical residues from the process generally remain on the sponge, and may be trapped within the enclosed pores or within remotely accessible pores.
To be effective as an energy storage device, a capacitor should have a high energy density (Watt-hours per mass) or high power density (Watts per mass). Conventional energy storage devices tend to have one, but not both, of these properties. For example, lithium ion batteries have energy densities as high as 100 Wh/kg, but relatively low power densities (1-100 W/kg). Examples of energy storage devices with high power density are RF ceramic capacitors. Their power densities are high, but energy densities are less than 0.001 Wh/kg. The highest energy capacitors available commercially are the electrochemical supercapacitors. Their energy and power densities are as high as 1 Wh/kg and 1,000 W/kg, respectively.
The dielectric film within the capacitor serves as the energy storage medium. Energy density is the amount of stored energy per unit volume of dielectric. To maximize the energy density of a capacitor, it is desirable to have a dielectric with a large surface per volume, a high dielectric constant, and a high dielectric strength. The energy density is a function of the dielectric constant and the dielectric strength, as follows:
Energy density=dielectricxc3x97(dielectric strength)2constantxe2x80x83xe2x80x83(3) 
A good capacitor geometry is one in which the dielectric is readily accessed electrically, that is, it has a low equivalent series resistance that allows rapid charging and discharging. High electrical resistance of the dielectric prevents leakage current. A good dielectric, therefore, has a high electrical resistance which is uniform at all locations. Additionally, a long-term stability (many charging-discharging cycles) is desired. Conventionally, dielectrics tend to become damaged during use.
The present invention provides a new and improved capacitor having an anode formed from a directionally-grown metallic sponge which provides high surface area and much improved energy storage capacity over conventionally known capacitors and a dielectric film with good electrical properties which has the ability of self repair in the event of a breakdown in the dielectric film.
In accordance with one aspect of the present invention, a capacitor is provided. The capacitor includes an anode formed from a directionally grown sponge having a high surface area. A dielectric film is formed on the surface of the sponge. A cathode includes an electrolyte in contact with the dielectric film.
In accordance with another aspect of the present invention, a directionally grown sponge suitable for forming capacitor anodes is provided. The sponge is formed by a method which includes growing dendrites on a substrate. The dendrites include an element selected from the group consisting of aluminum, antimony, arsenic, bismuth, cadmium, chromium, cobalt, copper, gallium, germanium, hafnium, iron, lead, magnesium, manganese, nickel, niobium, selenium, silicon, silver, tantalum, tellurium, thallium, tin, titanium, vanadium, zinc, zirconium, and alloys thereof.
In accordance with another aspect of the present invention, a method of forming a directionally grown sponge suitable for use as a capacitor anode is provided. The method includes directionally growing an electrically conductive sponge material on a substrate, such that the sponge material has a high surface area with few enclosed pores.
In accordance with another aspect of the present invention, a method of forming a capacitor is provided. The method includes forming an anode from an electrically conductive sponge material which has been directionally grown on a substrate and forming a dielectric film on a surface of the sponge.
One advantage of the present invention is that anodes with large surface areas per unit mass are formed, thereby enabling the size of the capacitor to be reduced.
Another advantage of the present invention is that it enables capacitors with energy densities of 10xe2x88x922 to 50 Wh/kg hours and power densities of 100,000 to 10,000,000 W/kg to be produced. For capacitors with liquid metal electrolytes, even higher power densities are possible.
Another advantage of the present invention is that titanium capacitors produced from directionally-grown anodes are suited to use in applications operating at frequencies between 20 Hz and 20,000 Hz.
Another advantage of the present invention arises from the ability to grow an efficient dielectric film on the anode surface, which is able to self-repair when damaged.